High-order soliton breakup and soliton self-frequency shifts in a microstructured optical fiber

Ultrashort pulse propagation in a polarization-maintaining microstructured fiber (with 1 μm core diameter and 1.1 m length) is investigated experimentally and theoretically. For an 80 MHz train of 130 fs pulses with average powers up to 13.8 mW launched into the lowest transverse mode of the fiber, the output spectra consist of discrete, multiple solitons that shift continuously to lower energies. The number of solitons and the amount that they shift both increase with the launched power. All of the data are quantitatively consistent with solutions of the nonlinear Schrödinger equation, but only when the Raman nonlinearity is treated without approximation, and self-steepening is included. These results remove any ambiguity as to the nature of these multiple solitons; they arise owing to the breakup of high-order solitons in the presence of nonlinear processes beyond self-phase modulation.